With an increase in the recording density of magnetic disks, the distance between the magnetic disk serving as a recording medium and the head for use in recording of information or playback has become almost nil close to contact therebetween. The magnetic disk is provided over the surface thereof with a carbon protective film or lubricant film for the purpose of diminishing abrasion due to the contact or sliding of the head thereon or preventing contamination of the disk surface.
The carbon protective film is produced generally by the sputtering process or CVD process. Since the disk surface is protected with the two films, i.e., the carbon protective film and the lubricant film thereover, the interaction between the carbon protective film and the lubricant is important.
The lubricants generally in use are perfluoropolyethers having functional groups. Examples of functional groups are hydroxyl, amino and cyclophosphazene group. More specific examples of lubricants include PHOSFAROL A20H, product of Matsumura Oil Research Corporation which is represented by the formula (II)
wherein n is 3 and Rf is —CF2CF2O(CF2CF2CF2O)zCF2CF2— (Patent Literature 1).
Particularly, lubricants having a phosphazene group are materials having high resistance to decomposition and known as materials for giving high durability to magnetic disks (for example, Patent Literature 2, 3). However, with reference to the lubricant of Patent Literature 2, claim 1 therein defines a compound wherein n is an integer of 1 to 5, whereas the preparation process of the patent concerned affords only a mixture which is not satisfactory in lubricating performance. With the lubricant of Patent Literature 3, a cyclophosphazene ring has a fluoropolyether main chain wherein the number of substituent is 1, and the lubricant is low in bonded ratio as listed in Table 1 given later.
Patent Literature 1: WO 2010/027096
Patent Literature 2: JP 2000-260017A
Patent Literature 3: JP 2004-352999A